1. Field of the Invention
The present invention relates to a process for preparing aluminum sulfate that is less colored from high purity sulfuric acid that is used in the electronic industries. More particularly, the present invention relates to a process for preparing less colored aluminum sulfate from high purity sulfuric acid that is used in the electronic industries, while preventing corrosion of storage tanks and pipings used for sulfuric acid and apparatus used for the preparation of aluminum sulfate.
2. Description of the Related Art
Aluminum sulfate is used as a water-purifying agent for water-supply and factory water or effluent, as a paper-sizing agent or a mordant, in fire-extinguisher, in the production of aluminum compounds or tanning of white leather, as a clarifier of oil and fat, as a catalyst for various reactions, and the like.
In general, aluminum sulfate is prepared by charging sulfuric acid into a reactor made of a material resistant to heat and acids, adding aluminum hydroxide in a stoichiometric or slightly excess amount, heating and reacting them. When solid aluminum sulfate is to be obtained, a concentrated aluminum sulfate solution is poured in a solidification bed and cooled till the solidification is completed, and a mass is comminuted and packaged.
When aluminum sulfate is to be obtained in a liquid form, the concentrated aluminum sulfate solution is diluted in the reactor, poured into a storage tank and cooled.
Some applications of aluminum sulfate require whiteness and clearness. Therefore, the aluminum sulfate solution obtained from the reaction is purified using a filtration apparatus that is precoated with diatomaceous earth, or is contacted with activated carbon to remove coloring materials by adsorption (see, for example, Japanese Patent Kokai Publication No. 87985/1975). In the above process, the organic materials (e.g. humic acid, etc.,) which are included in aluminum hydroxide as the raw material, are removed by filtration and/or adsorption to achieve decoloration.
In recent years, with the quick progress in the electronic industries, and in particular, with the rise in demand for high level processing techniques due to the increase of an integration level in the semiconductor industry, a large amount of high purity sulfuric acid is used in a chemical processing step and a rinsing step.
High purity sulfuric acid is replaced with a new batch in a short cycle to prevent problems caused by contamination with impurities.
Studies have been conducted to use the sulfuric acid that is discharged in a large amount from the electronic industries as the raw material in the preparation of surface acoustic wave resonator. However, it has been found that, when specific waste sulfuric acid discharged from the electronic industries is used, aluminum sulfate is colored yellow, and the yellow color cannot be removed even by the treatment with activated carbon.